Precipitation process for making polyvinylidene fluoride powder coatings and coatings made by the process

ABSTRACT

A process for making pigmented PVDF-based powder coatings by coagulation and powder based coatings made by the process. The process involves blending a PVDF latex and a water reducible acrylic polymer and pigment dispersion, adding coagulant to precipitate solid PVDF and pigment associated by ionic interaction with acrylic polymer binder, separating solids and drying the precipitate. Also, methods for coating substrates with the powder coating and coated substrates.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a process for making polyvinylidenefluoride (“PVDF”) powder coatings using a precipitation process inaqueous media. Specifically, the process involves the use of coagulationto precipitate PVDF and an acrylic resin blend from an aqueousdispersion prepared from blending PVDF latex and a water reducibleacrylic resin solution or dispersion. The invention also involvespigmented PVDF-based powder coating compositions made by the coagulationprocess.

[0003] 2. The Prior Art

[0004] PVDF coating compositions can be used for coating a large varietyof metal substrates, including aluminum, steel and galvanized steel, andhas wide acceptance as an architectural coating due to the properties ofPVDF, including high crystallinity and photo-oxidative resistance, whichprovide for coatings that are weather resistant and stand up tocorrosive or other harsh environments. In order to balance theperformance of PVDF, a secondary resin is normally needed to, amongother things, provide good adhesion to substrate and to reduce theshrinkage of the polyvinylidene fluoride due to excess crystallization.The secondary resin, which may be a thermoplastic polymer, enhances thefilm forming capabilities of the PVDF-based coating.

[0005] Commercial PVDF coating materials are generally solventdispersions and release solvent during coating preparation. Solventemission is generally managed by the end user through the installationand operation of an incineration system in a coating line. Theincinerator will burn the VOC of the solvent emission to reduce orpreclude harmful emission of VOC to the atmosphere. The need for anincinerator is, generally, a regulatory requirement. Thus, PVDF basedpaints in a solvent dispersion will require the end user to investcapital to install air pollution control apparatus, such as anincinerator, and incur costs to maintain the equipment and comply withregulatory requirements. These costs can be avoided by the use ofsolvent free coatings, such as PVDF-based powder coating preparations.

[0006] Additional benefits are obtained through use of solvent-free PVDFcoating materials, such as powder coatings. Powders can be sold andshipped in a ready to use state thereby obviating any need by the enduser for mixing, stirring or thinning during application. Powder basedcoatings can be used more efficiently than wet spray paint and thepowder coatings have a higher percentage usage because the powder can berecycled during coating operations. The high raw material utilizationassociated with powder coating arises from the lack of any solventrequirement for application. Also, because the powder has no VOC thereis significantly reduced absorption of energy during film forming on asubstrate after application thereby resulting in energy savings by theend user during application. These, and other, advantages of PVDF-basedpowder coating compositions result in consumer and industrial demand forsuch coating products.

[0007] Because of the benefits of powder based coatings, there isincreasing industrial and consumer demand for such coating compositions.Accordingly, the art is constantly evolving with novel processes formaking PVDF-based powder coatings. The art is also constantly seekingnovel PVDF-based coating materials with improved properties overexisting coating compositions and formulations.

[0008] For example, relatively recent processes are described in U.S.Pat. Nos. 4,770,939 and 5,346,727 for obtaining PVDF-based powdercoatings by cryogenic grinding of a melt extruded combination of PVDFand compatible thermoplastic. U.S. Pat. No. 5,229,460 describes agrinding process for making PVDF-based powder coatings that does notinvolve cryogenic conditions. A method for making a pigmented PVDFpowder coating using a solvent removal process of a solvent baseddispersion paint, but without coagulation of a polymer latex mixture, isdescribed in U.S. Pat. No. 5,739,202. A process for making a powdercomprising a (meth)acrylate polymer and fluoropolymer by combining twopolymer latex phases without coagulation is described in U.S. Pat. No.5,827,608. Some of the processes in the art are inefficient, and otherprocesses in the art do not result in a PVDF-based powder coating thathas acceptable pigmentation.

[0009] It was an object of the invention to develop an efficientcoagulation method for making pigmented PVDF-based powder coatingcompositions.

[0010] It was a further object of the invention to develop a coagulationprocess that results in a powder coating that has sufficientpigmentation to provide even coloring when heat cured after applicationto a substrate.

[0011] It was another object of the invention to develop a coagulationprocess for making PVDF-based powder coatings that reduces the need foradditives, such as surfactant, yet provide for an adequately pigmentedcoating.

[0012] It was another object of the invention to develop PVDF-basedpowder coating compositions having appropriate pigmentation and suitableproperties for industrial coatings.

[0013] These, and other objects of the invention, are achieved by theprocess described herein involving coagulation of PVDF latex and a waterreducible acrylic resin solution or dispersion. The aqueous acrylicresin comprises polymer chains containing side chain ionic moieties,which anchor onto the surface of pigment to form a stable acrylicpolymer/pigment dispersion which does not require a surfactant to form.The acrylic polymer/pigment dispersion and PVDF latex are precipitatedto obtain the pigmented powder coating composition. The pigmented powdercoating composition can be applied to any number of substrates and isheat cured forming a pigmented film over the substrate having even andfull color.

[0014] In the present Specification, all parts and percentages are byweight/weight unless otherwise specified.

SUMMARY OF THE INVENTION

[0015] The invention concerns coagulation of PVDF latex and a waterreducible acrylic polymer solution or dispersion, containing eithercationic or anionic species. The water reducible acrylic polymer phasefurther comprises pigment. The coagulation is induced by the addition ofacid or base coagulant. For anionic species, acid coagulant is used andfor cationic species, base coagulant is used. This coagulation processprecipitates a desired homogeneous solid blend. After solid separationand spray drying, a powder suitable for powder coating is produced.

[0016] The PVDF-based powder coating compositions are generally appliedto a substrate in powder form. The powder coated substrate is heated tofuse the PVDF and the water reducible polymer, preferably in the form ofan acrylic polymer/pigment dispersion, in a continuous film coating onthe substrate. In order to obtain the continuous film coating, the PVDFand water reducible acrylic polymer must be miscible during and afterfusion, and the water reducible acrylic polymer is selected for thepowder coating composition to have this miscibility.

DESCRIPTION OF THE DRAWING

[0017]FIG. 1 is a schematic illustrating the molecular interaction ofthe water reducible acrylic polymer, pigment and PVDF in embodiments ofthe invention where the water reducible acrylic polymer comprisesanionic species.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The term PVDF as used in the Specification pertains tohomopolymers of vinylidene fluoride and copolymers comprising vinylidenefluoride and other monomers having about 80% to about 99% vinylidenefluoride and from about 1% to about 20% other monomers, preferablyhexafluoropropylene. PVDF is produced by emulsion polymerization, theresult of which is a latex. The invention uses PVDF latex from emulsionpolymerization in the coagulation process for making a PVDF-based powdercoating. Commercially available PVDF latexes, such as those availablefrom Ausimont USA, Inc., Thorofare, N.J., U.S.A. under the tradenameHYLAR®, such as HYLAR MP3 PVDF, may be used in the invention.

[0019] As discussed above, PVDF coatings generally require a secondaryresin. The secondary resin must be selected such that it is misciblewith PVDF during and after fusion by heating the powder coating,generally after application on a substrate.

[0020] PVDF is miscible with a wide range of polymers such aspolymethacrylates, polyacrylates and polyvinyl acetates containing C═Ogroups in the ester, amide or ketone forms. The miscibility, in part,arises from the interaction between the C═O bonds of thepolymethacrylates, polyacrylates and polyvinyl acetates, as the case maybe, and the CH₂ groups of the vinylidene fluoride. For purposes of theinvention, water reducible polymer containing ionic species, eithercationic or anionic species, preferably anionic acrylic resins, are usedas the secondary polymers in the powder coating compositions.

[0021] A particularly preferred secondary polymer is a water reducibleform of polyalkyl (meth)acrylate, which is prepared by polymerization analkyl (meth)acrylate with (meth)acrylic acid to generate a waterreducible acrylic resin containing anionic species after neutralization.The water reducible polyalkyl (meth)acrylate may also be prepared bypolymerization of an alkyl (meth)acrylate with a minimized amount ofanother (meth)acrylate containing amine group, such asN,N-dimethyl-2-aminoethyl (meth)acrylate to provide cationic speciesafter neutralization with an acid. The content of comonomer with ionicspecies is minimized to an amount just enough to provide waterdispersion ability or reducibility. The amount ranges from about 1 toabout 20% of monomer content. The water reducible polyalky(meth)acrylate can also be prepared by polymerization of monomercombinations to give acrylic polymer with different performance.Examples of these are monomers selected from the group consisting ofmethyl methacrylate, ethyl methacrylate, methyl acrylate, ethylacrylate, butyl methacrylate, butyl acrylate, hydroxyethyl methacrylate,and hydroxyethyl acrylate, or combinations thereof

[0022] The PVDF-based powder coating composition is obtained by aprocess comprising the steps of (1) combining and blendingpolyvinylidene fluoride latex with a water reducible acrylic polymerphase which comprises an acrylic polymer/pigment dispersion which is thewater reducible acrylic polymer having ionic species, such as side chainionic moieties, in solution or dispersion in water with pigment, (2)coagulating the blend by addition of an acid or base to precipitate asolid mixture, (3) separating the solid precipitate from water, and (4)drying the solid precipitate. Grinding aids, fillers, and otheradditives to help coating performance can also be added before thecoagulation step. The water-reducible polymer may be either anionic orcationic.

[0023] The PVDF latex is obtained by emulsion polymerization whichgenerally involves use of anionic surfactant, such as water solublefluorosurfactants which may be in the form of a salt of perfluorinatedcarboxylic acid which have a general formula of X(CF₂)_(n) COO⁻M⁺, whereX is a hydrogen or fluorine, n is an integer from about 6 to about 20,preferably from about 8 to about 12 and M⁺ is an alkali metal ion andammonium ion. The PVDF latex used in the process of this invention willcomprise these types of surfactant.

[0024] The water reducible acrylic polymer acts as a dispersing aid forthe pigment. The pigment interacts with the acrylic polymer because ofmolecular attraction between the highly polar pigment surface and theion species, such as the side chain ionic moieties, of the acrylic. Theionic attraction helps to bind the pigment to the polymer, such asanchoring on to the surface of the pigment, and provides for the pigmentto precipitate out with the polymer during the precipitation process.Generally, a surfactant is needed to provide pigment with gooddispersion ability in an aqueous medium, however, the invention does notinvolve use of additional surfactant to allow the pigment to form auniform dispersion with the PVDF latex. After precipitation from thecoagulation process, undesired emulsifiers in the PVDF latex mixture areremoved.

[0025] Any type of pigments may used in the invention. Preferredpigments are, or will comprise, one or more of the following: titaniumdioxide which is available from Whittaker, Clark & Daniels, SouthPlainfield, N.J., U.S.A.; Arctic blue #3, Topaz blue #9, Olympic blue#190, Kingfisher blue #211, Ensign blue #214, Russet brown #24, Walnutbrown #10, Golden brown #19, Chocolate brown #20, Ironstone brown #39,Honey yellow #29, Sherwood green #5, and Jet black #1 available fromShepard Color Company, Cincinnati, Ohio, U.S.A.; black F-2302, blueV-5200, turquoise F-5686, green F-5687, brown F-6109, buff F-6115,chestnut brown V-9186, and yellow V-9404 available from Ferro Corp.,Cleveland , Ohio, U.S.A. and METEOR® pigments available from EnglehardIndustries, Edison, N.J., U.S.A.

[0026] Additives, such as chemicals helpful in coating formation or flowpromoters, may be incorporated with either the water reducible acrylicpolymer, or the PVDF emulsion, prior to coagulation. Examples of theseadditives are Lanco™ Flow P10 available from Lubrizol, Wickliffe, Ohio,U.S.A. and MODAFLOW® Powder available from Solutia, St. Louis, Mo.,U.S.A. In addition, the introduction of anionic moieties to acrylicpolymer may increase the water sensitivity of coating during service.Also, acrylic polymer may be prepared from crosslinkable monomer such ashydroxyethyl methacrylate. Therefore, a crosslinker, such as melamineformaldehyde resin, carbodiimide crosslinker or hydroxyalkyl amide, maybe added to the system to improve coating performance.

[0027] The PVDF latex and the water reducible acrylic polymer phase arecombined in a vessel capable of blending the two, such as a mixer orreactor. The PVDF latex comprises PVDF, surfactant and water, and thewater-reducible polymer phase comprises the acrylic polymer having ionicspecies, preferably side chain ionic species, and pigment in adispersion form. In addition, either the PVDF latex or the acrylicpolymer phase can comprise additives and/or fillers. The contents of thevessel are blended for a period of time sufficient to completely combinethe PVDF latex and water reducible acrylic polymer phase, preferablyfrom about 15 minutes to about 30 minutes.

[0028] After the blending is complete, a coagulant is added to theblend. The coagulant serves to destabilize the blend and causes theprecipitation of the solids. Because of the ionic species of the waterreducible polymer with the pigment, the acrylic polymer precipitatesassociating with pigment. During coagulation the acrylic polymerprovides good interaction with the PVDF particles. This results in PVDFand pigment particles bound to solid acrylic polymer in the precipitate.

[0029] The choice of coagulant is dependant on the ionic species on theacrylic polymer. If the acrylic polymer comprises anionic species, acidcoagulant is generally used, and if the acrylic polymer comprisescationic species, basic coagulant is used. Examples of acid coagulantsare inorganic acids, such as nitric acid, hydrochloric acid, sulfuricacid, and the like, and combinations thereof. Examples of basiccoagulants are ammonium hydroxide, sodium hydroxide, potassiumhydroxide, sodium carbonate, potassium carbonate, and the like, andcombinations thereof.

[0030] Now referring to FIG. 1 which shows the molecular aspects of thecoagulation in an embodiment of the invention wherein the waterreducible acrylic polymer phase comprises anionic species and acidcoagulant is used. The acrylic polymer phase, which also can be referredto as the pigment dispersion, shown generally as 1, comprises acrylicpolymer 2 in water reduced form having one or more anions 3, which maybe side chain anionic moiteties, and pigment 4 having high surfacepolarity which is the form of a dispersion. The anions 3 of the acrylicpolymer 2 interact with the high surface polarity of the pigment 4 suchthat the acrylic polymer 2 and the pigment 4 become associated by ionicinteraction. The PVDF latex is shown generally in FIG. 1 as 6 andcomprises PVDF 7 having one or more anions 8. The coagulant 9 formssolid precipitate material shown generally on the right side of thearrow in FIG. 1 as 10. When the coagulant 9 is added, the anions 3 ofthe acrylic polymer 2 in the water reducible acrylic polymer phase 1which are not associated with pigment 4 are neutralized by action of thecoagulant and the acrylic polymer 2 with associated pigment 4precipitates from the water as pigment dispersed in an acrylic polymerbinder, shown generally in FIG. 1 as 11. Likewise, the anions 8 of thePVDF 7 are neutralized by action of the coagulant 9 and the PVDF 7precipitates as a solid dispersed in the neutralized acrylic polymer. Aswould be understood by one skilled in the art, the solid precipitatedmaterials may also comprise solid additives and fillers eitherseparately, associated with each other, associated with the acrylicpolymer 2 and/or associated with the PVDF 7. If a cationic acrylicpolymer is used, then the pigment should be selected to have an anioniccharge in water to achieve the desired interaction between the acrylicpolymer and pigment prior to coagulation.

[0031] The precipitate material is dried by means that would beappreciated by one skilled in the art, preferably by spray drying orfiltering. Once the precipitate is dried, the PVDF-based powder coatingcomposition is generally formed. Optionally, however, the driedprecipitate may be ground to form the powder coating composition.

[0032] The powder coating compositions comprising PVDF and the pigmentdispersed in the acrylic polymer binder can be applied to a substrate byany suitable means for achieving an even distribution of the particles.In particular, the powder can be applied by electrostatic sprayapplication apparatus, whereby charged particles are sprayed onoppositely charged substrates. Other useful application techniquesinclude, but are not limited to, cloud chambers, fluidized beds and thelike. Such techniques will be understood by one skilled in the art andneed not be described further.

[0033] After the coating is applied to the substrate, the coatedsubstrate is then heated to a temperature between about 200° C. andabout 280° C. to cure the coating by forming a continuous film. Duringthe film forming process the PVDF and acrylic polymer become miscibleand fuse in the form of the film. Because the pigment is well dispersedin the acrylic polymer binder, the pigment is generally evenly anduniformly distributed over the substrate. The acrylic polymer thusfunctions as a dispersion aid and no dispersion aid dedicated todisperse the polymer, such as surfactant, is necessary for the pigmentedPVDF-based powder coatings of the invention.

EXAMPLE

[0034] 500 grams of methy ethyl ketone solvent were placed into a oneliter glass reactor equipped with a mechanical stirrer, a thermometer, acondenser and a dropping funnel. After heating the solvent to 75° C., 15grams of benzoyl peroxide was added to the reactor. A mixture ofuninhibited monomers comprising 284.7 grams of methyl methacrylate and15.3 grams of methacrylic acid was slowly added to the reactor inincrements of about 10 millimeters every 3 minutes, for a total of 90minutes. The reaction between the methyl methacrylate and methacrylicacid was allowed to proceed an additional 90 minutes at 75° C. underagitation to obtain water reducible form of PMMA. The solution waspermitted to cool to room temperature.

[0035] 150 grams of the water reducible PMMA was added to a 16 ounceglass jar. The water reducible PMMA was partially neutralized with 5.9grams of 5M ammonium hydroxide under agitation using a stirrer. Aftermixing, 109 grams of deionized water was added slowly to the glass jarunder agitation. After obtaining a homogeneous solution of PMMA anddeionized water, 52.5 grams of titanium dioxide (U.S.P. grade fromWhittaker, Clark & Daniels) and 300 grams of soda-lime silicate glassbeads having a diameter of about 3.0 millimeters from QuakenbushCompany, Crystal Lake, Ill., U.S.A., were placed into the glass jar andthe jar was sealed. The contents of the glass jar were then mixed byvigorous shaking by a mechanical shaker for 2 hours. The beads wereremoved by filtration to obtain an acrylic polymer phase comprisingwater reducible PMMA and titanium dioxide pigment in the form of adispersion.

[0036] 107 grams of the acrylic polymer phase was blended with a PVDFlatex (HYLAR MP3 PVDF from Ausimont) containing 24.95 weight percent ofthe polymer under a slow agitation. 3.3 grams of concentrated nitricacid was added to the blend of PVDF and the water reducible acrylicpolymer phase under slow agitation to coagulate the blend and theresulting precipitate was filtered and dried to form a powder. Thepowder was sprayed electrostatically onto a chromated aluminum substratewith a polarity about 30 to 40 KV and baked at 260° C. for 15 minutes. Afilm with 3 to 5 mil of thickness was formed. The film was observed tohave even and full color.

1. A process for making a powder coating composition comprising thesteps of blending a PVDF latex and a water reducible acrylic polymerphase having a water reducible acrylic polymer with ionic species and apigment, adding coagulant to form a precipitate, separating theprecipitate from water and drying the precipitate.
 2. The process ofclaim 1 wherein the PVDF is a homopolymer of vinylidene fluoride or acopolymer comprising vinylidene fluoride and other monomers.
 3. Theprocess of claim 2 wherein the copolymer comprises about 80% to about99% vinylidene fluoride and about 1% to about 20% hexafluoropropylene.4. The process of claim 1 wherein the water reducible acrylic polymer isa water reducible form of polyalkyl (meth)acrylate.
 5. The process ofclaim 4 wherein the polyalkyl (meth)acrylate is prepared bypolymerization of monomers selected from the group consisting of methylmethacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, butylmethacrylate, butyl acrylate, hydroxyethyl methacrylate, hydroxyethylacrylate and combinations thereof.
 6. The process of claim 1 wherein theionic species is anionic and the coagulant is an acid.
 7. The process ofclaim 6 wherein the coagulant is an inorganic acid.
 8. The process ofclaim 7 wherein the inorganic acid is selected from the group consistingof nitric acid, hydrochloric acid, sulfuric acid and combinationsthereof.
 9. The process of claim 1 wherein the ionic species is cationicand the coagulant is a base.
 10. The process of claim 9 wherein the baseis selected from the group consisting of ammonium hydroxide, sodiumhydroxide, potassium hydroxide, sodium carbonate, potassium carbonateand combinations thereof.
 11. The process of claim 1 wherein flowpromoters are incorporated into the PVDF latex or water reducibleacrylic polymer phase prior to blending.
 12. The process of claim 1wherein the water reducible acrylic polymer and pigment have oppositeionic moieties and are associated by ionic interaction.
 13. The processof claim 1 wherein the pigment comprises titanium dioxide.
 14. A powdercoating composition made by the process of claim
 1. 15. A powder coatingcomposition comprising PVDF and a pigment dispersed in an acrylicpolymer binder wherein the pigment and acrylic polymer associate byionic interaction.
 16. The composition of claim 15 wherein the PVDF is ahomopolymer of vinylidene fluoride or a copolymer comprising vinylidenefluoride and other monomers.
 17. The composition claim 16 wherein thecopolymer comprises about 80% to about 99% vinylidene fluoride and about1% to about 20% hexafluoropropylene.
 18. The composition of claim 15wherein the acrylic polymer is polyalkyl (meth)acrylate.
 19. Thecomposition of claim 18 wherein the polyalkyl (meth)acrylate is preparedby polymerization of monomers selected from the group consisting ofmethyl methacrylate, ethyl methacrylate, methyl acrylate, ethylacrylate, butyl methacrylate, butyl acrylate, hydroxyethyl methacrylate,hydroxyethyl acrylate and combinations thereof.
 20. The composition ofclaim 15 wherein the pigment comprises titanium dioxide.
 21. Thecomposition of claim 15 further comprising flow promoters.
 22. A methodfor coating a substrate comprising a) providing a substrate and a powdercoating composition comprising PVDF and a pigment dispersed in anacrylic polymer binder wherein the pigment and acrylic polymer associateby ionic interaction, b) applying the powder coating composition to thesubstrate and c) heating the coated substrate to a temperature of about200° C. to about 280° C.
 23. The method of claim 22 wherein the PVDF isa homopolymer of vinylidene fluoride or a copolymer comprisingvinylidene fluoride and other monomers.
 24. The method of claim 22wherein the acrylic polymer is polyalkyl (meth)acrylate.
 25. The methodof claim 24 wherein the polyalkyl (meth)acrylate is prepared bypolymerization of monomers selected from the group consisting of methylmethacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, butylmethacrylate, butyl acrylate, hydroxyethyl methacrylate, hydroxyethylacrylate and combinations thereof.
 26. The method of claim 22 whereinthe pigment comprises titanium dioxide.
 27. A coated substrate made bythe method of claim 20.